Self-supporting and liquid developable electro-photographic element



Jan. 16, 1968 s. HAZEN 3,364,021

S. SELF-SUPPORTING AND LIQUID DEVELOPABLE ELECTHOPHDTOGRAPHIC ELEMENT Filed Oct. 9, 1964 5/W00 TH SURFA CE Z//vc OXIDE 90//0 v/lvm CHLOE/015 Vl/VYL ACETATE COPOL YMEE "Zl/VC OX/OE- 70/30 STY/iENE BUTAD/E/VE COPOLYME/P ZII'VC OXIDE- 90/l0 VINYL CHLORIDE- V/NYL ACETATE COPOLYMER STANLEY s. HAZEA/ INVENTOR,

ATTORNEYS United States Patent C) 3,364,021 SELF-SUPPURTlN G AND LIQUID DEVELOPABLE ELECTRO-PHOTOGRAPHIC ELEMENT Stanley S. Hazen, Hilton, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Fiied Oct. 9, 1964, Ser. No. 402,798 Claims. (Cl. 961.8)

This invention relates to an improved self-supporting, reusable and liquid developable, composite electrophotographic (xerographic) element having exceptionally good electrical characteristics and high solvent resistance.

In electrophotography, it is customary to use a support material such as a metal plate or paper having one or more photoconductive layers thereon which, in the dark or in red light, will accept and hold a blanket electrostatic charge, for example, from a corona discharge source. On exposure of the charged element or plate to modulated light, for example, as by projection of a photographic image thereon, the charge is retained in the nonexposed areas of the plate, but discharged wholly or in part, depending on the intensity of illumination, in the exposed areas thereby producing an electrostatic image. This image is then rendered visible as a colored or pigmented powder image, advantageously by liquid development, for example, as described in K. A. Metacalfe et al.,

Australian Patent No. 212,315, published June 13, 1957. The resultant powder image may be used in a number of ways, for example, as a matrix for transfer imagewise of the powder or toner to a receiving sheet such as paper and then fused thereon with heat to provide a stable reproduction of the original subject. While such prior art processes have produced worthwhile reproductions, they have not proven entirely satisfactory for certain applications requiring especially high quality prints. For example, ordinary zinc oxide-resin photoconductive layers on paper support, even with the advantageous liquid development show several detrimental features, in particular (1) a tendency for developer powder or toner to become entrapped in the somewhat matte surface of the material and thereby discolor the background, and (2) the tendency for the usual binder resins, e.g., styrene-butadiene to become softened by the conventional solvents, e.g., cyclonexane, ordinary paint thinner, trichlorotrifluoroethane (Freon 113), etc., in which the toner particles for liquid development are dispersed, thereby producing some distortion of the image and prohibiting reuse of the element. While it is possible to overcome in some degree the unwanted retention of toner particles by providing the zinc oxide-resin photoconductive layer with an extremely smooth surface, e.g., by coating a conventional zinc oxideresin composition onto a plain cellophane sheet, and after drying, wetting the cellophane so that it readily strips from the composite sheet leaving a self-supporting, smooth-surfaced xerographic element, this method still has not overcome the disadvantageous tendency of the binder resins to become unduly softened on liquid development or toning. Accordingly, various attempts have been made to produce solvent developable xerographic layers or elements having greater resistance to solvents, for example, by replacing the conventional styrenebutadiene copolymeric resin binders with other more solvent resistant resinous materials. However, such proposed elements, though of fairly good solvent resistance, have not proven entirely satisfactory primarily because of 10W initial potential and poor charge retention.

I have now found that tough and flexible, reusable, self-supporting and liquid developable, composite electrophotographic elements combining both unexpectedly good electrical characteristics and high resistance to the carrier solvents used in liquid toner developing compositions can be prepared by overcoating a conventional photoconductive layer or core comprising a mixture of photoconductive zinc oxide and a resinous styrene-butadiene copolymer, on both surfaces thereof, with a photoconductive composition which dries to a solvent resistant layer, said layer comprising a mixture of photoconductive zinc oxide and certain resinous vinyl chloride-vinyl acetate copolymers wherein the vinyl chloride is the major component. The above-described novel three-layer element charges to the unexpectedly high voltage of 700-800 volts, whereas the core layer by itself charges to but 500' volts, and as for the overcoating layers, these by themselves cannot ordinarily be made to hold a charge above about 100' volts each, which is minimal for xerographic toning. It will be apparent that two-layer elements employing the same core layer, but with the solvent resistant coating on only one surface can also be prepared by the process of the invention. However, I have further found that such elements though chargeable to an operable voltage, but of substantially less than 700-800 volts, require extreme care in applying the liquid developer to just the solvent resistant surface, which fact greatly limits the utility of the two-layer element. This serious shortcoming is completely overcome in the three-layer element of my invention.

It is, accordingly, an object of the invention to provide a new and unexpectedly eiiicacious composite electrophotographic element comprising a core layer of photoconductive zinc oxide intimately dispersed in a resinous styrene-butadiene copolymer of certain composition range, with outer layers on both surfaces of a solvent resistant composition of photoconductive zinc oxide intimately dispersed in a resinous vinyl chloride-vinyl acetate copolymer of certain composition range. Another object is to provide a new three layer electrophotographic element which readily charges to a negative potential of 700-800, or more, volts. Another object is to provide a process for preparing such elements. Other objects will become apparent from a consideration of the description and the example.

In accordance with my invention, I prepare my new composite electrophotographic elements preferably by a coating technique involving three successive coatings over one another employing a preformed hydrophilic support material, e.g., an untreated cellophane sheet. In the first step, a dispersion or dope (A) comprising photoconductive zinc oxide, a resinous copolymer consisting of from 70-95% by weight of vinyl chloride and 30-5% by weight of vinyl acetate, but preferably from -95% and 15-5% of vinyl chloride and vinyl acetate, respectively, and a solvent for the resin such as certain chlorinated hydrocarbons and ketones, e.g., dioxane, mesityl oxide, methylisobutyl ketone, etc., is coated directly on the cellophane sheet to form layer 10 of the accompanying drawing, and dried. In the second step, a dispersion or dope (B) comprising photoconductive zinc oxide, a resinous copolymer consisting of from 60-80% by weight of styrene and 40-20% by weight of butadiene, but preferably from 65-75% and 35-25% of styrene and butadiene, respectively, and a solvent for the resin such as benzene, toluene, etc., is coated over above layer 10 to form layer 11 of the accompanying drawing, and dried. In the third step, another portion of the previously used dispersion or dope (A) is coated over layer 11 to form layer 12 of the accompanying drawing. Advantageously, the cellophane support sheet is held rigidly on a vacuum plate during the entire coating operations. Final curing of the structure formed as above is carried. out by heating the vacuum plate to a temperature of about -95 F. for a period of time sufficient to bring about satisfactory curing. In the final step, the cellophane supported element is immersed in water and the cellophane support stripped therefrom, leaving a three-layer element, i.e., an element having a core layer 11 of photoconductive zinc oxide dispersed in the specified resinous styrenebutadiene copolymer and outer solvent resistant layers 10 and 12 of photoconductive zinc oxide dispersed in the specified resinous vinyl chloride-vinyl acetate copolymer. The smooth-surfaced outer layer formed in contact with the cellophane support is especially suitable as the image recording layer in the xerographic process. An advantageous structure is one that has an over-all thickness of from 110 mils, and preferably about from 2-5 mils, the core layer accounting for from 4060% and each of the outer layers for from 20-30% of this thickness. The coating technique is adjusted by well-known methods to produce this result.

The proportions of solvents to solids in the above coating compositions (A) and (B) can vary widely just so a smooth flowing, homogeneous dispersion or dope is obtained, but preferably in the range of from 4060% by weight of solvent and from 60-40% by weight of solids. The ratio of zinc oxide to resin can vary from about 2:1 to 4:1. The particle size of the zinc oxide is preferably on the order of less than 0.5 micron mean diameter. Suitable photoconductive zinc oxides are commercially available under a variety of trade names such as White Seal No. 7, Green Seal No. 8 or XX78 (New Jersey Zinc Com- P u)- Since an ordinary layer of photoconductive zinc oxide is primarily sensitive to the ultraviolet region of the spectrum, i.e., an electrostatic charge on the layer can be discharged only by wavelengths of lights in the ultraviolet region, I have found it especially advantageous to incorporate optical sensitizing dyes in the layers of my electrophotographic elements to extend the sensitivity throughout the visible spectrum. Some of the well known dyes which are efficacious sensitizers for this purpose include acridine orange, fluorescein, eosin, methylene blue and rose bengal. These dyes can he used alone or in combinations of two or more in one or more layers of my composite element in the approximate amounts of from 0.0010.0l% based on the weight of the zinc oxide. Advantageously, the dyes are added in the form of their methanol solutions to the coating compositions and the resulting mixture thoroughly blended.

The following example further illustrates the manner of practicing my invention.

Example Dispersion (B) for making the center or core layer of the element illustrated by this example was of the following composition:

Toluene grams 400 1 In 20 cc. methanol.

The dispersion was prepared by mixing the toluene and zinc oxide together at a high shearing rate in a Waring Blendor for 10 minutes, then the dyes in methanol were added with continued mixing for 5 minutes at a low shearing rate, and then the Pliolite S-7 solution was added to the mixture and the dispersion blended for an additional 5 minutes at a high shearing rate. The resulting dispersion had a solids content of about 30 percent by weight, wherein the ratio of zinc oxide to resin was 3: 1, and was used directly to make the core layer as described in the procedure below.

Dispersion (A) for making the outer solvent resistant layers of the element was prepared in similar fashion to that described for dispersion (B) above, except that the liolite S7 was replaced with 60 g. of Vinylite VYl-IH (a 10% by weight vinyl chloride-vinyl acetate copolymer) in 400 g. of methylisobutyl ketone instead of in toluene.

In preparing the element, dispersion (A) was coated to a wet thickness of 8 mils on a plain-untreated, nonwater resistant, 1.5 mil cellophane support being held rigidly on a vacuum plate. After solvent evaporation, the second or core layer, using dispersion (B), was coated over the first layer to a wet thickness of 10 mils. After solvent evaporation of prior core layer, the third layer using another portion of dispersion (A) was coated over the core layer to a wet thickness of 10 mils. The resulting structure, dried on the vacuum plate heated to 90-95 F., was immersed in water and the cellophane support was stripped from the first layer, leaving a three-layer element (illustrated by the accompanying drawing) having one smooth surface which was used for the image recording layer.

The element produced as above described was charged under a corona charger on the smooth surface to a negative potential of from 700800 volts, exposed in an imagewise manner, and developed by immersion in a liquid developer comprising an intimate mixture of approximately equal parts of Spirit Nigrosine (CI 50415) and polystyrene in Sohio Odorless Solvent #3440 (Standard Oil Company of Ohio), in the proportions of about from l5 parts of the solids and the remainder to make a total of parts of the solvent. A receiving sheet of bond paper was placed into contact with the resulting powder image (matrix) and the back side of the paper was subjected briefly to a negative corona discharge to facilitate transfer of the powder image to the paper. The paper print was then separated from the matrix and heated to a temperature sufiicient to fuse the powder image firmly to the paper base. The positive print thus obtained was a clear and well defined reproduction of the original image. The matrix or element was ready for reuse after wiping clean with a cotton pad. This process was carried out with this same element, without registering the exposures, more than 100 times with no measurable image degradation and essentially no increase in background density.

The accompanying drawing is a sectional view illustrating the structure of the element of the invention, and more particularly the element of the above example.

In the drawing, layers 1% and 12 each represents a photoconductive layer comprising a mixture of zinc oxide and a 90/ 10% by weight vinyl chloride-vinyl acetate copolymer, layer 10 having a very smooth surface, while layer 11 represents a photoconductive layer comprising a mixture of zinc oxide and a 70/ 30% by weight styrenebutadiene copolymer.

The liquid developers that are useful in the practice of the invention include many Well known developer compositions. To obtain sufficient fineness of the pigment and resin particles, the usual practice is to ball mill the ingredients in concentrated form and to disperse the desired amount of the concentrate in a suitable carrier solvent. For example, the liquid developer employed in the above example illustrating the invention was prepared by first fusing a mixture of the polystyrene and the Nigrosine, cooling the fused mixture, reducing it to a powder, and then ball milling it with about an equal amount of the carrier solvent (Sohio Odorless Solvent #3440 available from Standard Oil Company of Ohio), until the desired particle size was obtained. This concentrate was in the form of a viscous liquid which was readily dilutable with the same carrier solvent to give the desired composition of final liquid developer. It will be understood, however, that other carrier solvents can be substituted for the above-mentioned carrier solvent such as for example normal hcptane, normal hexane, petroleum ether and various commercial petroleum solvents having volatility in the kerosene to gasoline range, Shell 140 solvent and Shell 8230 base (both available from the Shell Chemical Corporation), carbon tetrachloride, tetrachloroethylene, trichloroethylene, etc. Also pigments other than Nigrosine and binders other than polystyrene can be used with the above carrier solvents, e.g., Cyan Blue Toner GT (R. H. Wisinall, Jr., US. Patent No. 2,- 486,351, issued October 25, 1949, Benzidine Yellow, Sudan Red (CI 26100), Hausa Yellow G (CI 11680), Pyrazolone red and maroon pigments, carbon blacks, etc.; and diphenyls, chlorinated biphenyls and polyphenyls, ester gums, gum mastic, stearic acid, resinous polysiloxanes, paraflins, Piccolytes (those having melting points from 50-l35 C., a thermoplastic terpene resin available from Pennsylvania Industrial Chemical Corp.), etc.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

What I claim is:

1. An electrophotographic element comprising a core layer (a) comprising photoconductive zinc oxide and a resinous copolymer consisting of from 60-80% by weight of styrene and from 40-20% by weight of butadiene, overcoated on each side with a solvent resistant layer (b) comprising photoconductive zinc oxide and a resinous copolymer consisting of from 7095% by weight vinyl chloride and from 30-5% by weight of vinyl acetate.

2. An electrophotographic element of claim 1 wherein the ratio of said photoconductive zinc oxide and said resinous copolymer in each of said layers is from 2:1 to 4:1.

3. An electrophotographic element of claim 1 wherein each of said layers contains at least one sensitizing dye extending the spectral sensitivity of said photoconductive zinc oxide in each of said layers into the visible spectrum.

4. An electrophotographic element of claim 1 wherein the overall thickness of said element is from 1-10 mils, of which the core layer (a) constitutes from -60% of said thickness and each of the outer layers constitute from 2030% of said thickness.

5. An electrophotographic element of claim 1 wherein at least one surface is extremely smooth.

6. An electrophotographic element of claim 1 wherein the said copolymer of layer (a) is a copolymer consisting of approximately by Weight of styrene and 30% by weight of butadiene, and wherein the said copolymer of layer (b) is a copolymer consisting of approximately by weight of vinyl chloride and 10% by weight of vinyl acetate.

7. An electrophotographic element of claim 6 wherein the ratio of said photoconductive zinc oxide and said resinous copolymer in each of said layers is from 2:1 to 4:1.

8. An electrophotographic element of claim 6 wherein each of said layers contains at least one sensitizing dye extending the spectral sensitivity of said photoconductive zinc oxide in each of said layers into the visible spectrum 9. An electrophotographic element of claim 6 wherein the overall thickness of said element is from 1-10 mils, of which the said core layer (a) constitutes 40-60% of said thickness and each outer layer (b) constitutes from 20 30% of said thickness.

10. An electrophotographic element of claim 6 wherein one surface is extremely smooth.

References Cited UNITED STATES PATENTS 2,987,395 6/1961 Jarvis 96l.8

NORMAN G. TORCHIN, Primary Examiner. 

1. AN ELECTROPHOTOGRAPHIC ELEMENT COMPRISING A CORE LAYER (A) COMPRISING PHOTOCONDUCTIVE ZINC OXIDE AND A RESINOUS COPOLYMER CONSISTING OF FROM 60-80% BY WEIGHT OF STYRENE AND FROM 40-20% BY WEIGHT OF BUTADIENE, OVERCOATED ON EACH SIDE WITH A SOLVENT RESISTANT LAYER (B) COMPRISING PHOTOCONDUCTIVE ZINC OXIDE AND A RESINOUS COPOLYMER SONSISTING OF FROM 70-95% BY WEIGHT VINYL CHLORIDE AND FROM 30-5% BY WEIGHT OF VINYL ACETATE. 